Actin Polymerization: Riding the Wave

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<ul><li><p>Actin Polymerization: Riding the Wave</p><p>Laurie G. Smith1 and Rong Li2</p><p>WAVE/SCAR has long been known to activate theactin-nucleating Arp2/3 complex in a Rac-dependentmanner. Recent biochemical and genetic studies haverevealed important roles for four WAVE-associatedproteins in regulating WAVE function.</p><p>Proper execution of a wide variety of actin-dependentprocesses in eukaryotic cells depends on tight controlover the timing and location of actin polymerization.As a nucleator of actin polymerization, the highlyconserved Arp2/3 complex plays a central role in thecellular control of actin dynamics [1,2]. The Arp2/3complex by itself has little effect on actin polymeriza-tion, but the complex can be activated by a variety ofproteins that couple intracellular or extracellular cuesto the control of actin nucleation [1,2]. The most inten-sively studied Arp2/3 complex activators are membersof the WASP/WAVE family, which are regulated in partby Rho-type small GTPases. Purified WASP protein isinactive because of autoinhibition, which is relieved bybinding to Cdc42GTP and phosphatidylinositol 4,5-bisphosphate (PIP2) [1,2]. In contrast, purified WAVE isfully active [3], but in vivo WAVE activity is dependenton RacGTP [4]. </p><p>A mechanism for Rac regulation of WAVE activitywas revealed in a seminal recent study [5] showing thatWAVE1 protein isolated from bovine brain extractsbelongs to multiprotein complex that is inactive at stim-ulating Arp2/3-dependent actin polymerization in vitro.Upon addition of RacGTP or the SH2/SH3 domainprotein Nck, WAVE1 is released from the complex in anactive form (Figure 1). Here we discuss a series of morerecent studies [69] demonstrating important roles forWAVE complex components in the regulation of actinpolymerization in vivo. The new results are largely inagreement with the model illustrated in Figure 1, andshed additional light on the regulation of WAVE bythese associated proteins.</p><p>PIR121, Nap125 and Abi2: Proteins with a SplitPersonalityIn vitro, RacGTP and Nck stimulate dissociation of theWAVE complex into two subcomplexes: one comprisingWAVE1 and HSPC300; and the other comprising theRac-associated protein PIR121/Sra-1 [10], the Nckassociated protein Nap125 [11] and Abelson tyrosinekinase-interacting protein Abi2 [12] (Figure 1). The factthat WAVE is activated after dissociation from the latterthree proteins implies that they are negative regulatorsof WAVE activity. Recent genetic studies provide </p><p>evidence of this and for additional roles for PIR121,Nap125 and Abi2 in WAVE regulation in vivo.</p><p>WAVE was originally discovered in Dictyostelium asthe product of the scar gene: scar mutant cellsdisplay reduced size and motility, as well as reducedF-actin labeling [13]. Recently, a null mutation in theDictyostelium gene encoding PIR121, pirA, has beenshown to cause a phenotype essentially opposite tothat of scar mutant cells [6]: pirA mutant cells areabnormally large and display motility defects associ-ated with excess pseudopod formation. Quantitativeanalysis showed an almost two-fold increase in F-actin content (relative to total actin) in pirA mutantscompared to wild-type cells. Significantly, doublemutant scar pirA mutant cells have the same pheno-type as scar single mutants, indicating that PIR121acts on WAVE/SCAR to inhibit its function [6].</p><p>Drosophila has a single gene, scar, encoding aWAVE homolog. Genetic studies have demonstratedthat this gene is critical for a wide variety of actin-dependent processes throughout development [14].Null mutations in the kette gene, which encodes theDrosophila homolog of Nap125, cause defects in theactin-dependent processes of neurite growth andglial cell migration; these defects are associated withthe formation of disorganized, excess F-actin bundles[15]. Recently, the glial cell migration defect in kettemutants was shown to be suppressed in flies hemizy-gous for the scar gene, demonstrating that Kette actsto repress SCAR function [7]. </p><p>Unexpectedly, other studies have yielded resultsimplying that PIR121/Sra-1, Kette/Nap125 and Abi2can play positive, rather than inhibitory, roles inWAVE/SCAR-mediated actin polymerization. In cul-tured Drosophila S2 cells, downregulation by RNAinterference (RNAi) of Sra-1, Kette or Abi inhibitedlamellipodium formation and led to loss of cortical F-actin, the same phenotype observed after RNAi ofSCAR [8,9]. Interestingly, the Sra-1(RNAi), Kette(RNAi)or Abi(RNAi) cells showed a substantial decrease inlevels of SCAR protein [8,9]. Similarly, DictyosteliumSCAR protein was reduced to barely detectable levelsin pirA mutant cells [6].</p><p>These observations reveal an unforeseen function forthe WAVE complex in regulating WAVE/SCAR proteinlevels. Treatment of Abi(RNAi) cells with proteasomeinhibitors partially restored SCAR protein levels, sug-gesting that Abi protects SCAR protein from protea-some-mediated degradation [9]. Degradation of SCARprotein in Sra-1(RNAi), Kette(RNAi) or Abi(RNAi) cellswould seem to be sufficient explanation for the similar-ity of their phenotypes to that of SCAR(RNAi) cells, butadditional observations suggest that Abi functions tolocalize SCAR protein at the cortex as well as to protectit from degradation [9].</p><p>How can the results of these RNAi studies bereconciled with the apparently contradictory resultsfrom analyses of Drosophila and Dictyosteliummutants discussed earlier? The answer may lie in thefact that RNAi does not completely eliminate target</p><p>Dispatch</p><p>Current Biology, Vol. 14, R109R111, February 3, 2004, 2004 Elsevier Science Ltd. All rights reserved. DOI 10.1016/j.cub.2004.01.016</p><p>1Section of Cell and Developmental Biology, University ofCalifornia San Diego, 9500 Gilman Drive, La Jolla, California92093, USA. 2Department of Cell Biology, Harvard MedicalSchool, 240 Longwood Avenue, Boston, Massachusetts02115, USA.</p></li><li><p>proteins, whereas the kette and pirA mutationsanalyzed are null alleles. If the remaining SCAR proteinin Kette(RNAi), Sra-1(RNAi) and Abi(RNAi) cells ispresent in intact WAVE/SCAR complexes, then theseRNAi treatments may reduce F-actin polymerization byreducing levels of SCAR without relieving it fromdependence on Rac/Nck-mediated activation. In con-trast, the small amounts of SCAR protein remaining inpirA null mutants may be constitutively active in thecomplete absence of PIR121 protein, causing excessactin polymerization. Alternatively, differences in thecellular context might explain why reduction or loss ofPIR121/Sra-1 or Kette seems to stimulate actinpolymerization in some cases and inhibit it in others.</p><p>HSPC300: Another Brick in the WallRelatively few studies have addressed the function ofHSPC300. The observation that this protein remainsassociated with WAVE following Rac or Nck-mediateddissociation of the WAVE complex does not clearlypredict what role HSPC300 might play in WAVEregulation or actin polymerization. RNAi of HSPC300in Drosophila S2 cells produced a mild and variablephenotype resembling that seen in SCAR(RNAi) cells[9]. Earlier work [16], however, showed that, in maize,null mutations in the gene for the HSPC300 homologBRICK1 cause a failure in the formation of localizedcortical F-actin enrichments in dividing and expandingleaf epidermal cells. In the absence of these F-actinenrichments, the epidermal cells fail to undergospecific cell shape changes and to become polarizedin preparation for asymmetric cell divisions [16]. </p><p>These findings suggest that BRICK1 has a positiverole in the regulation of a plant Arp2/3 complexactivator. The Arp2/3 complex itself is present inplants, and has recently been shown to play an impor-tant role in epidermal cell morphogenesis [17]. Genesencoding PIR121 and Nap125 homologs have alsobeen identified in Arabidopsis, though they remain tobe functionally characterized [17]. But the Arp2/3complex activator that BRICK1, PIR121 and Nap125presumably interact with remains to be identified. Inany case, results from the analysis of BRICK1 in plantsand HSPC300 in Drosophila S2 cells suggest thatHSPC300 plays an important role in WAVE activation,localization and/or stabilization.</p><p>Concluding PerspectivesGenetic analyses of WAVE complex components havecome together with biochemical studies to enhance our</p><p>understanding of how this complex participates in theregulation of WAVE. In particular, these studies haverevealed an interesting, dual role for Nap125/Kette,PIR121/Sra-1 and Abi in the regulation of WAVE func-tion. These proteins appear to promote WAVE functionby protecting it from degradation and perhaps alsohelping to localize it correctly, while at the same timeinhibiting WAVEs activity. Linking WAVE activation toits degradation in this way might help to ensure thatWAVE activation is very tightly coupled to appropriateactivating signals.</p><p>Many questions remain concerning the functions ofWAVE complex components and regulation of thepathway in which they function. One obvious questionis why does reduction or loss of PIR121, Nap125, andAbi2 lead in some cases to excess F-actin accumula-tion and in others to loss of F-actin? If the WAVEcomplex is activated by RacGTP at the plasma mem-brane, what keeps WAVE there once it dissociates fromthe complex and Rac? Answers to these questionsmight reveal new facets of WAVE regulation. </p><p>Further questions concern the roles of other proteinsimplicated in WAVE regulation. One of these is IRSp53,which binds to WAVE2 and enhances its activation ofArp2/3-dependent actin polymerization in vitro [18].Moreover, IRSp53 is implicated in Rac-induced mem-brane ruffling and can directly link WAVE2 to RacGTPin vitro, suggesting a role for IRSp53 in Rac regulationof WAVE in vivo [18]. A more recently identified WAVE-interacting protein is WRP, a Rac-selective GTPaseactivating protein. Binding to WAVE has been proposedto direct WRP to Rac, which could attenuate WAVEactivation by stimulating Racs GTPase activity, orcould enhance WAVE activation by recycling Rac fromthe PIR121Nap125Abi2 complex. Understanding howthe contributions of these WAVE-interacting proteinsare integrated to achieve proper regulation of WAVE invivo is an important challenge that lies ahead.</p><p>References1. Welch, M.D., and Mullins, R.D. (2002). Cellular control of actin</p><p>nucleation. Annu. Rev. Cell Dev. Biol. 18, 247-288.2. Pollard, T.C., and Borisy, G.G. (2003). Cellular motility driven by</p><p>assembly and disassembly of actin filaments. Cell 112, 453-465.3. Machesky, L.M., Mullins, R.D., Higgs, H.N., Kaiser, D.A., Blanchoin,</p><p>L., et al. (1999). Scar, A WASP-related protein, activates nucleationof actin filaments by the Arp2/3 complex. Proc. Natl. Acad. Sci. USA96, 3739-3744.</p><p>4. Miki, H., Suetsugu, S., and Takenawa, T. (1998). WAVE, a novelWASP-family protein involved in actin reorganization induced byRac. EMBO J. 17, 6932-6941.</p><p>5. Eden, S., Rohatgi, R., Podtelejnikov, A.V., Mann, M., and Kirschner,M.W. (2002). Mechanism of regulation of WAVE1-induced actinnucleation by Rac1 and Nck. Nature 418, 790-793.</p><p>DispatchR110</p><p>Figure 1. A model for Rac/Nck-mediatedactivation of WAVE/SCAR.</p><p>In the presence of RacGTP or Nck, theinactive WAVE complex dissociates intotwo subcomplexes: one containingWAVE1 and HSPC300, which is active instimulating Arp2/3-dependent actin poly-merization, and one containing PIR121,Nap125 and Abi2 [5]. Hypothetical inter-actions between RacGTP and PIR121,and between Nck and Nap125 are also</p><p>illustrated, based on the previous identification of PIR121 as a Rac-associated protein [10] and of Nap125 as an Nck-associatedprotein [11]. Alternative names for PIR121, Nap125, WAVE, and HSPC300 used in this article are shown in red.</p><p>Inactive</p><p>RacGTPNck</p><p>WAVE</p><p>HSPC300</p><p>Active</p><p>+</p><p>Abi2</p><p>Nap125</p><p>PIR121</p><p>BRICK1</p><p>Sra-1</p><p>Kette</p><p>SCAR</p><p>Current Biology</p></li><li><p>6. Blagg, S.L., Stewart, M., Sambles, C., and Insall, R.H. (2003). PIR121regulates pseudopod dynamics and SCAR activity in Dictyostelium.Curr. Biol. 13, 1480-1487.</p><p>7. Bogdan, S., and Klmbt, C. (2003). Kette regulates actin dynamicsand genetically interacts with Wave and Wasp. Development 130,4427-4437.</p><p>8. Rogers, S.L., Wiedemann, U., Stuurman, N., and Vale, R.D. (2003).Molecular requirements for actin-based lamella formation inDrosophila S2 cells. J. Cell Biol. 162, 1079-1088.</p><p>9. Kunda, P., Craig, G., Dominguez, V., and Baum, B. (2003). Abi, Sra1,and Kette control the stability and localization of SCAR/WAVE toregulate the formation of actin-based protrusions. Curr. Biol. 13,1867-1875.</p><p>10. Kobayashi, K., Kuroda, S., Fukata, M., Nakamura, T., Nagase, T.,Nomura, N., Matsuura, Y., Yoshida-Kubomura, N., Iwamatsu, A.,and Kaibuchi, K. (1998). P140SRA-1 (specifically Rac1-associatedprotein) is a novel specific target for Rac small GTPase. J. Biol.Chem. 273, 291-295. </p><p>11. Kitamura, T., Kitamura, Y., Yonezawa, K., Totty, N.F., Gout, I., Hara,K., Waterfield, M.D., Sakaue, M., Ogawa, W., and Kasuga, M. (1996).Molecular cloning of p125Nap1, a protein that associates with anSH3 domain of Nck. Biochem. Biophys. Res. Commun. 219, 509-514.</p><p>12. Daiz, Z., and Pendergast, A.M. (1995). Abi-2, a novel SH3-contain-ing protein interacts with the c-Abl tyrosine kinase and modulatesc-Abl transforming activity. Genes Dev. 9, 2569-2582.</p><p>13. Bear, J.E., Rawls, J.F., and Saxe, C.L. III (1998). SCAR, a WASP-related protein, isolated as a suppressor of receptor defects in lateDictyostelium development. J. Cell Biol. 142, 1325-1335.</p><p>14. Zallen, J.A., Cohen, Y., Hudson, A.M., Cooley, L., Wieschaus, E.,and Schejter, E.D. (2002). SCAR is a primary regulator of Arp2/3-dependent morphological events in Drosophila. J. Cell Biol. 156,689-701.</p><p>15. Hummel, R., Leifker, K., and Klambt, C. (2000). The DrosophilaHEM-2/NAP1 homolog KETTE controls axonal pathfinding andcytoskeletal organization. Genes Dev. 14, 863-873.</p><p>16. Frank, M., and Smith, L.G. (2002). A small, novel protein highly con-served in plants and animals promotes the polarized growth anddivision of maize leaf epidermal cells. Curr. Biol. 12, 849-853. </p><p>17. Deeks, M.J., and Hussey, P.J. (2003). Arp2/3 and the shape ofthings to come. Curr. Opin. Plant Biol. 6, 561-567.</p><p>18. Miki, H., Yamaguchi, H., Suetsugu, S., and Takenawa, T. (2000).IRSp53 is an essential intermediate between Rac and WAVE in theregulation of membrane ruffling. Nature 408, 732-735.</p><p>19. Soderling, S.H., Binns, K.L., Wayman, G.A., Davee, S.M., Ong, S.H.,Pawson, T, and Scott, J.D. (2002). The WRP component of theWAVE-1 complex attenuates Rac-mediated signaling. Nat. Cell Biol.4, 970-975.</p><p>Current BiologyR111</p></li></ul>